This application is the US national phase of International Application No. PCT/GB95/02808, filed Dec. 1, 1995, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an imaging system.
2. Discussion of Prior Art
In order for a mammalian eye to be able to observe objects over a wide range of light intensity levels, a mammalian retina comprises five general types of neurons organised into a two layer system. A first layer, the outer plexiform layer, consists of detector, horizontal and bipolar neurons. It has been shown that the detector neurons"" response is a logarithmic function of the photon flux. Signals from the detector neurons are passed to the horizontal neurons which allow the signals to spread laterally. The bipolar neurons respond to the differences between the signals from the detector and horizontal neurons. The result is a bipolar neuron response which is a function of the spatial and temporal changes in the response of the detector neurons and the response is relatively insensitive to changes in the overall illumination level.
Electronic sensors such as charge coupled device sensors generally operate over a limited range of illumination levels. Commercial camera systems often incorporate an automatic gain control to alter the operating point depending on the level of illumination. Where there is a bright light source within a scene, the automatic gain control results in the loss of detail in the scene.
In a natural scene, the light intensity L(x,y) reflected from an object having a reflection coefficient R(x,y) under an illumination level I(x,y) is given by the equation:
L(x,y)=I(x,y)R(x,y).
Within any scene, the illumination level may typically vary by up to two decades whilst the reflectivity might vary by up to one decade, giving a maximum of three decades of variation in intensity. In order to detect intensity changes at a level of one per cent over an intensity range of three decades after digitisation of the sensor output, approximately seventeen bits per pixel would be required. In contrast, when the image is displayed, for example using a cathode ray tube, a human operator can only distinguish approximately one hundred grey levels, which corresponds to about seven bits. Large amounts of information are therefore lost.
A logarithmic detector response D(x,y), is given by the equation,
D(x,y)=log(R(x,y))+log(I(x,y)).
If a spatial average response is subtracted from the detector response, this will have the effect of attenuating the contribution from the I(x,y) term more than that from the R(x,y) term. A retina which attenuates the intensity contribution due to the illumination level relative to the contribution due to the reflection coefficient reduces the dynamic range required to represent a scene without losing the critical information represented by R(x,y). In the case of the mammalian eye, the outer plexiform layer converts a large dynamic range input into a smaller dynamic range output.
Sensors which mimic the behaviour of mammalian retinas are sometimes referred to as artificial retinas. Artificial retina structures have been reported previously. An artificial retina structure has been described by C. A. Mead in xe2x80x9cAnalog VLSI and Neural Systemsxe2x80x9d, Addison Wesley, 1989 and illustrated by FIG. 1. The Mead structure suffered from two disadvantages. The structure had a slow response time and so was limited to bright scenes and mismatch between devices meant that only objects with a relatively high contrast could be observed. In addition, the number of devices within each pixel meant that the smallest possible size of 109 xcexcm by 97 xcexcm was relatively large, limiting the spatial resolution of the structure.
K. A. Boahen and A. G. Andreou, in Int. J. Comp. Vision, Volume 8 (1992), pages 764 to 772, proposed a retina with fewer devices per pixel. This retina comprised two networks of lateral devices connected within each pixel with a small feedback loop. The Boahen and Andreou retina suffered from the disadvantage that corrections required to overcome device variations were illumination dependent.
United Kingdom Patent Application No. 9204434.6 describes a differential amplifier incorporating floating gate devices which are programmable to correct imbalances arising from device mis-match. The devices therein are arranged to operate above a device threshold voltage such that the devices are operating in saturation.
French published patent specification FR-A-2 543 363 describes an analogue integrated circuit using transistors whose threshold voltage is electrically adjustable, together with a comparator suitable for controlling the adjustment of the potentials on the floating gates of two floating gate devices. The transistors of the analogue circuit are arranged to operate above the threshold voltage, where the current in the transistor is directly related to the threshold voltage. For a field effect transistor operating above the threshold voltage, the current through the transistor, ID is given by the expression:
ID=xcex2(Vgsxe2x88x92VT)2
where xcex2 and VT, the threshold voltage, are device dependent parameters. Adjusting the threshold voltage of a floating gate transistor may reduce the effect of this device dependent parameter but differences between devices of xcex2 still remain. The teaching of FR-A-2 543 363 indicate that field effect transistors are not suitable for operation below the threshold voltage, referring to two distinct modes, a xe2x80x9cblocked modexe2x80x9d (un mode bloquxc3xa9) and a xe2x80x9cconductive modexe2x80x9d (un mode conducteur), the threshold voltage separating the two modes.
Analogue image processing was reviewed by S. Collins and M. Wade in a scientific paper presented to an IEE Symposium xe2x80x9cIntegrated Image Sensors and Processingxe2x80x9d on Dec. 5th 1994. In this paper, the suitability of an imaging system comprising a logarithmic detector and a centre-surround receptive field for use as a surveillance system was discussed. This paper indicated that such a system would be advantageous since a typical scene has an intensity histogram which exhibits a multi-modal distribution whereas a processed image would have a uni-modal distribution, which could be more efficiently encoded as a digital signal. The paper did not provide an example of such a system nor any indication as to how one may be constructed. The results presented therein were the results of a computer modelling exercise.
It is an object of the invention to provide an alternative imaging system.
The present invention provides an imaging system incorporating
(i) detecting means incorporating multiple detector elements arranged to detect electromagnetic radiation from a scene and to generate pixel image signals in response thereto; and
(ii) processing means for filtering the pixel image signals spatially and for generating processed image output signals having a dynamic range which is less than that of radiation from the scene,
characterised in that
(iii) each detector element is incorporated in a respective pixel circuit arranged to provide a pixel image signal which is substantially a logarithmic function of incident radiation intensity and has reduced dynamic range in comparison thereto;
(iv) the processing means is spatially separate from the detecting means and is arranged to provide high-pass spatial filtering; and
(v) the pixel circuits and the processing means are arranged in combination to selectively counteract the illumination dependence of pixel image signals and to provide an edge-enhanced representation of the imaged scene.
Separating the detecting means from the processing means and incorporating means for generating pixel image signals whose dynamic range is less than that of an image intensity at a respective pixel and substantially a logarithmic function of the image intensity at the pixel provides the advantages that:
(a) the size of pixels within the detecting means may be reduced compared with the Mead arrangement, with consequently greater resolution;
(b) the system is responsive to a greater range of radiation received thereat and provides enhanced dynamic range reduction in addition to dynamic range reduction provided by spatial filtration performed in the processing means; and
(c) processing means dynamic range performance requirements are relaxed by performing dynamic range compression in the detecting means.
The system may incorporate means for outputting a digitised filtered image signal such that each pixel image signal is presented by not more than five bits in the digitised signal. This provides data which retains essential image information and is recordable without demanding considerable digital storage memory capacity.
The imaging system may include electronically programmable correcting means for correcting for variations in pixel response to radiation incident thereupon. Variations in pixel elements may, if uncorrected, reduce the sensitivity of an imaging system. Correcting pixel non-uniformities may increase the sensitivity of an imaging system.
The correcting means may comprise a floating-gate field-effect transistor configured to provide a correction dependent upon charge stored in one or more of its floating gates. This provides a practical configuration for correcting variations in pixel response where the detecting means incorporates a large number of pixels.
The imaging system of the invention may form the basis for an electronic photographic system where processed images are stored and then later displayed.